Thermoplastic resin composition having improved impact resistance

ABSTRACT

A thermoplastic resin composition comprising: a graft copolymer obtained by polymerizing 10 to 70 parts by weight of a monomer containing 60 to 100% by weight of at least one vinyl monomer selected from the group consisting of a (meth)acrylate ester compound, an aromatic vinyl compound and a vinyl cyanide compound and 0 to 40% by weight of a monomer which is copolymerizable therewith, in the presence of 30 to 90 parts by weight of hollow rubber particles which are rubber obtained by polymerizing a polymerization component comprising a butadiene without using a crosslinking agent and have a porosity of 3 to 90% in the form of a latex, wherein the total amount of the monomer and the rubber particles is 100 parts by weight; and a thermoplastic resin.

RELATED APPLICATIONS

This application is a nationalization of PCT Application No.PCT/JP02/03833 filed Apr. 17, 2002. This application claims priorityfrom Japanese Patent Application No. 2001-120783 filed on Apr. 19, 2001.

TECHNICAL FIELD

The present invention relates to a resin composition comprising arubber-containing graft copolymer and a thermoplastic resin, with theresin composition having excellent impact resistance.

BACKGROUND ART

Various conventional proposals have been made in order to improve theimpact resistance of thermoplastic resins. For example, it is known thata copolymer containing a diene rubber or an acrylate rubber isincorporated in a vinyl chloride resin (JP-B-39-19035). In addition, forimproving impact resistance, a process of increasing the particle sizeof a rubber component (JP-B-42-22541) and a process for lowering theglass transition temperature (hereinafter referred to as “Tg”) of arubber component (JP-A-2-1763, JP-A-8-100095) are proposed.

A technique was recently proposed for improving the impact resistance ofa thermoplastic resin by using a graft copolymer having a Tg of 0 degreeC. or less, containing 0.1 to 5% by weight of a crosslinking agent andcontaining a hollow rubber having a porosity of 3 to 90% in the form ofa latex (WO00/02963).

However, the above-described processes are accompanied by such problemsas a marked increase in the raw material cost, and insufficientimprovement in a graft copolymer containing a butadiene rubber which hasbeen most popularly used for the improvement of impact resistance.

Specifically, when the amount of the crosslinking agent is too small andbecomes less than 0.1% by weight, the rubber particles are collapsed andfinely dispersed upon molding or forming. As a result, no stressconcentration occurs and the effects of improving impact resistance arenot obtained.

Generally, a rubber-containing copolymer such as described above ismixed for improving impact resistance of a thermoplastic resin such as avinyl chloride resin or the like. The stress concentration of a moldedarticle and generation and expansion of voids in the rubber play animportant role. For stress concentration, it is necessary to introduce arubber component having a modulus much lower than that of thethermoplastic resin. In practice, various rubbers have been introducedand the size or shape of the rubber component has been optimized. It ispredicted that generation and expansion of voids in a rubber contributesignificantly to the growth of a shear yield to permit a large energyabsorption amount upon impact test and would be expected to lead to animprovement of the impact resistance of a rubber-containingthermoplastic resin.

Therefore, it would be very important to learn how to accelerategeneration and expansion of voids in the rubber component. Generationand expansion of voids in the rubber component upon impact on a moldedarticle (under stress) depend much on the crosslinked condition of therubber. When the rubber component is made hollow in advance, expansionof voids would easily proceed under stress.

DISCLOSURE OF THE INVENTION

The present inventors studied the relationship among void (hollow) stateof the rubber component in the form of a latex, the hollow state of amolded article of a thermoplastic resin having arubber-component-containing impact resistance modifier incorporatedtherein, and impact resistance of the molded article, while changing theamount of the crosslinking agent which controls the crosslinked state ofthe rubber component.

The present inventors have succeeded in preparing a resin compositionwhich is excellent in impact resistance and which comprises arubber-containing graft copolymer and a thermoplastic resin. The graftcopolymer is obtained by polymerizing 10 to 70 parts by weight of amonomer composed of 60 to 100% by weight of at least one vinyl monomerselected from the group consisting of (meth)acrylate ester compounds,aromatic vinyl compounds and vinyl cyanide compounds, and 0 to 40% byweight of a monomer which is copolymerizable therewith. The graftcopolymer is polymerized in the presence of 30 to 90 parts by weight ofhollow rubber particles which are obtained by polymerizing, without acrosslinking agent, a polymerization component comprised primarily ofbutadiene and have a porosity of 3 to 90% in the form of a latex. Therubber containing graft copolymer is compounded with the thermoplasticresin.

The present inventors considered that it is important to realize stressconcentration by introducing a rubber component having a modulus ofelasticity much lower than that of a thermoplastic resin, such as avinyl chloride resin, to be a continuous phase of a molded article. As aresult, they have found that when a butadiene monomer is used as arubber component, it is preferable that the content of the crosslinkingagent for the rubber component is smaller; and when the crosslinkingagent is not contained, the rubber particles become void upon impact(under stress) and impact resistance is most effectively improved. Thus,the present invention has been completed.

Specifically, the present invention relates to

a thermoplastic resin composition which includes a graft copolymerobtained by polymerizing 10 to 70 parts by weight of a monomercontaining 60 to 100% by weight of at least one vinyl monomer selectedfrom the group consisting of a (meth)acrylate ester compound, anaromatic vinyl compound and a vinyl cyanide compound and 0 to 40% byweight of a monomer which is copolymerizable therewith, in the presenceof 30 to 90 parts by weight of hollow rubber particles which are rubberobtained by polymerizing a polymerization component comprising abutadiene without using a crosslinking agent and have a porosity of 3 to90% in the form of a latex, wherein the total amount of the monomer andthe rubber particles is 100 parts by weight. The composition includesthermoplastic resin wherein the thermoplastic resin is a vinyl chlorideresin comprising 50% by weight or more of vinyl chloride. Finally, inthe resin composition the hollow rubber particles are obtained by using0.5 to 20% by weight of a hydrophilic seed polymer having a particlesize less than 0.04 μm.

BEST MODE FOR CARRYING OUT THE INVENTION

Hollow rubber particles can be synthesized by various processes.Examples of a well known process include (a) polymerization of a monomerof the O layer in an W/O/W emulsion (O: oleophilic, W: hydrophilic), (b)swelling of core-shell particles having an expandable core at Tg or moreof the shell layer to thereby make the inside of the particles hollow,(c) two-stage polymerization of polymers different in solubilityparameter, (d) preparation of an O/W emulsion by finely dispersing, inwater, a polymerizable monomer containing a crosslinkable monomer and ahydrophilic monomer and an oily substance, and removal of the oilysubstance by polymerization of the monomer; and (e) a process usingtransfer of a carboxylic acid which has been copolymerized in theparticles, under acid or alkali conditions (Application of SyntheticLatex, by Takaaki Sugimura, et al., p. 285). The rubber particles of thepresent invention having a hollow in the form of a latex can be preparedby any one of these processes.

In the present invention, a butadiene monomer containing no crosslinkingagent is polymerized with a seed polymer having a low molecular weightand a certain level of hydrophilicity to obtain a hollow rubber latexhaving rubber particles filled, inside thereof, with water. Then theresulting rubber latex is polymerized with 10 to 70 parts by weight of amonomer mixture containing 60 to 100% by weight of at least one vinylmonomer selected from the group consisting of (meth)acrylate estercompounds, aromatic vinyl compounds and vinyl cyanide compounds and 0 to40% by weight of a monomer copolymerizable therewith to thereby obtain agraft copolymer. A resin composition comprising the resulting graftcopolymer and a thermoplastic resin, which has an excellent impactresistance, is obtained.

The seed polymer has used in the present invention has, as a skeleton,rubber such as diene rubber, acrylic rubber, silicone rubber, olefinrubber or the like; a semi-rigid polymer such as a butylacrylate-styrene copolymer, ethyl acrylate-styrene copolymer or thelike, or a rigid polymer such as a styrene-methyl methacrylate copolymeror the like, and contains a chain transfer agent such ast-dodecylmercaptan, n-dodecylmercaptan or the like. A small amount ofacrylic or methacrylic acid may be incorporated therein in order tocontrol its hydrophilicity.

The seed polymer preferably has a particle size less than 0.04 μm.Particle sizes of 0.04 μm or more make it difficult to heighten theporosity of the hollow butadiene rubber in the form of a latex. Althoughthere is no lower limit, a particle size of about 0.005 μm is the lowestlimit which can be measured. The seed polymer is preferably added in anamount of 0.5 to 20% by weight based on 100% by weight of the hollowrubber particles including the seed polymer. Amounts less than 0.5% byweight make it difficult to heighten the porosity of the hollowbutadiene rubber in the form of a latex, while those exceeding 20% byweight dilute the butadiene rubber to thereby lower its impactresistance improving effects.

In the hollow butadiene rubber usable in the present invention, thepolymerization component having butadiene as a main component except aseed polymer may be 100% by weight butadiene or a copolymer of butadienewith 50% by weight or less of a non-crosslinkable copolymerizable vinylmonomer. The butadiene is preferably 70% by weight to 100% by weight,more preferably 90% by weight to 100% by weight, based on the totalamount of the polymerization component except a seed polymer.

It is preferable that the rubber is a rubber elastomer having a Tg of 0degree C. or less, and that the Tg is lower. Examples thereof satisfyingsuch conditions include butadiene rubber, styrene-butadiene rubber,acrylonitrile-butadiene rubber and the like.

The void of the hollow butadiene rubber in the form of a latex can beconfirmed by embedding the rubber latex in an epoxy resin or the like,dyeing it with ruthenium tetraoxide or the like and observing it withTEM. The porosity can be calculated by precisely determining theparticle size of the rubber latex by “Microtrac UPA” and then measuringthe light scattering intensity of the same rubber latex. The porosity ofthe hollow rubber in the form of a latex is 3 to 90%, preferably 10 to60%, from the viewpoint of the impact resistance improving effects ofthe end molded article. At a porosity exceeding 90%, the rubberparticles is apt to collapse upon molding or forming and impactresistance cannot be improved stably. At a porosity less than 3%, on theother hand, generation and enlargement of voids in the rubber do notproceed smoothly upon impact and impact resistance improving effects ofthe end molded article are not sufficient.

In order to allow the end molded article to exhibit the maximum impactresistance improving effects, it is preferred to set the optimumparticle size of the graft copolymer of the present invention within0.05 to 2.0 mu·m, though it varies depending on the kind of thethermoplastic resin. Particle sizes outside the above-described rangetend to lower the impact resistance improving effects.

Although the synthesizing method of the hollow butadiene rubber is notparticularly limited, it can be synthesized efficiently by emulsionpolymerization.

The graft copolymer of the present invention is obtained by polymerizing10 to 70 parts by weight, preferably 12 to 40 parts by weight, of amonomer in the presence of 30 to 90 parts by weight, preferably 60 to 88parts by weight, of a hollow butadiene rubber component. Amounts of thehollow rubber component less than 30 parts by weight do not bring aboutsufficient impact resistance improving effects, while those exceeding 90parts by weight cause collapse of the particles of the impact resistancemodifier upon formation of a molded article containing it to therebylower the impact resistance improving effects.

The monomer to be polymerized in the presence of the hollow rubberparticles is a monomer or monomer mixture containing 60% by weight ormore of at least one selected from a (meth)acrylate ester, an aromaticvinyl compound, a vinyl cyanide compound and vinyl chloride. Examples ofthe (meth)acrylate ester include methyl methacrylate, ethylmethacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, methylacrylate, ethyl acrylate, butylacrylate, 2-ethylhexyl acrylate and thelike. Examples of the aromatic vinyl compound include styrene,alpha-methylstyrene, chlorostyrene and the like. Examples of the vinylcyanide compound include acrylonitrile, methacrylonitrile and the like.

Examples of another copolymerizable monomer include a (meth)acrylateester, other than the above-described (meth)acrylate esters, such asglycidyl (meth)acrylate and the like, and a maleimide compound such asmaleimide, N-phenylmaleimide and the like.

Examples of the thermoplastic resin of the present invention include avinyl chloride resin, an acrylic resin, a styrene resin, a carbonateresin, an amide resin, an ester resin, an olefin resin and the like.Examples of the vinyl chloride resin include polyvinyl chloride, acopolymer containing at least 50% by weight of vinyl chloride and amonomer such as vinyl acetate, ethylene or the like which iscopolymerizable with vinyl chloride, a chlorinated vinyl chloride resinand the like. Examples of the acrylic resin include poly(methylmethacrylate), a copolymer containing at least 50% by weight of methylmethacrylate and a monomer, such as methyl acrylate, butyl acrylate,styrene or the like, which is copolymerizable with methyl acrylate, andthe like. Examples of the styrene resin include polystyrene, astyrene-acrylonitrile copolymer, an alpha-methylstyrene-acrylonitrilecopolymer, a styrene-maleimide copolymer, astyrene-alpha-methylstyrene-acrylonitrile copolymer, astyrene-alpha-methylstyrene-maleimide-acrylonitrile copolymer, astyrene-maleic anhydride copolymer and the like. Examples of thecarbonate resin include a bisphenol polycarbonate, an aliphaticpolycarbonate and the like. Examples of the amide resin include nylon 6,nylon 6-6, nylon 12 and the like. Examples of the ester resin includepolyethylene terephthalate, polybutylene terephthalate and the like.Examples of the olefin resin include polypropylene, polyethylene, cyclicpolyolefin and the like. Based on 100 parts of any one of thesethermoplastic resins, 1 to 50 parts by weight of the graft copolymercontaining a hollow rubber is added. Among the above-describedthermoplastic resins, a vinyl chloride resin have markedly high impactresistance improving effect.

The present invention will hereinafter be described in further detailbased on Examples. However, the present invention is not limitedthereto.

EXAMPLE 1

After mixing 200 parts by weight of water and 30 parts by weight ofsodium oleate, the mixture was heated to 70 degree C. After itstemperature reached 70 degree C., nitrogen substitution was conducted. Amixture of 9 parts by weight of butyl acryolate, 1 part by weight ofacrylonitrile and 3 parts by weight of t-dodecylmercaptan was thenadded. Thirty minutes thereafter, 0.5 part by weight (as a solidcontent) of a 2% aqueous potassium persulfate solution was added,followed by polymerization for 1 hour. A mixture of 81 parts by weightof butyl acrylate, 9 parts by weight of acrylonitrile and 27 parts byweight of t-dodecylmercaptan was continuously added over 3 hours. Twohours thereafter, polymerization was conducted to obtain seed latex(S-1) having an average particle size of 0.015 mu·m. In a pressurepolymerizer, 2 parts by weight (as a solid content) of seed latex (S-1),0.4 part by weight of tripotassium phosphate, 0.2 part by weight of Nasalt of a beta-naphthalinesulfonic acid-formalin condensate, 0.016 partby weight of ferrous sulfate (FeSO₄ 7H₂O), 0.04 part by weight ofethylenediaminetetraacetic acid 2Na salt and 0.5 part by weight ofsodium oleate were mixed. To the resulting mixture were added 98 partsby weight of butadiene. After the liquid temperature was adjusted to 40degree C., 0.2 part by weight of paramethane hydroperoxide and 0.4 partby weight of sodium formaldehyde sulfoxylate were added, followed bypolymerization at 40 degree C. Each of 2 hours and 5 hours after theinitiation of the polymerization, 0.7 part by weight of sodium oleatewas added, each of 2 hours and 7 hours after the initiation of thepolymerization, 0.2 part by weight of paramethane hydroperoxide wasadded, and polymerization was conducted for 20 hours. As a result,hollow butadiene rubber latex (R-1) having a porosity of 50% andparticle size of 0.08 mu·m was obtained.

After heating 77.5 parts by weight (as a solid content) of rubber latex(R-1) to 60 degree C., 0.0016 part by weight of ferrous sulfate(FeSO₄.7H₂O), 0.004 part by weight of ethylenediaminetetraacetic acid2Na salt and 0.2 part by weight of sodium formaldehyde sulfoxylate wereadded, and a mixture of 16.5 parts by weight of methyl methacrylate, 3parts by weight of butyl acrylate, 3 parts by weight of styrene and 0.01part by weight of cumene hydroperoxide over 3 hours was continuouslyadded. Post polymerization was conducted for 1 hour to obtain graftcopolymer latex (G-1) having an average particle size of 0.9 mu·m.

The graft copolymer latex (G-1) was coagulated with hydrochloric acid,followed by heat treatment, dehydration and drying, to thereby obtainpowdery graft copolymer (A-1).

In a blender, 8 parts by weight of graft copolymer (A-1), 2 parts byweight of dioctyl tin mercaptide, 0.8 part by weight of polyol ester,0.2 part by weight of diol ester of montanic acid and 100 parts byweight of a vinyl chloride resin (average polymerization degree: 700)were mixed to thereby obtain a powdery resin composition. The resultingresin composition was kneaded for 5 minutes by a roll of 160 degree C.,followed by pressurization for 10 minutes by a press at 190 degree C.,to thereby obtain a molded article of 5.0 mm thick. From the moldedarticle, a test piece of JIS NO.2 A for Izod impact resistance test wasmade. Izod impact resistance of the test piece was measured and resultsare shown in Table 1.

EXAMPLE 2

After mixing 200 parts by weight of water and 7 parts by weight ofsodium oleate, the mixture was heated to 70 degree C. After itstemperature reached 70 degree C., nitrogen substitution was conducted. Amixture of 9 parts by weight of butyl acrylate, 1 part by weight ofacrylonitrile and 3 parts by weight of t-dodecylmercaptan was thenadded. Thirty minutes thereafter, 0.5 part by weight (as a solidcontent) of a 2% aqueous potassium persulfate solution was added,followed by polymerization for 1 hour. A mixture of 81 parts by weightof butyl acrylate, 9 parts by weight of acrylonitrile and 27 parts byweight of t-dodecyl mercaptan was continuously added over 3 hours. Postpolymerization was conducted for 2 hours to obtain seed latex (S-2)having an average particle size of 0.025 mu·m. In a pressurepolymerizer, 2 parts by weight (as a solid content) of seed latex (S-2),0.4 part by weight of tripotassium phosphate, 0.2 part by weight of Nasalt of a beta-naphthalinesulfonic acid-formalin condensate, 0.016 partby weight of ferrous sulfate (FeSO₄ 7H₂O), 0.04 part by weight ofethylenediaminetetraacetic acid. 2Na salt and 0.5 part by weight ofsodium oleate were mixed. To the resulting mixture were added 98 partsby weight of butadiene. After the liquid temperature was lowered to 40degree C., 0.2 part by weight of paramethane hydroperoxide and 0.4 partby weight of sodium formaldehyde sulfoxylate were added, followed bypolymerization at 40 degree C. Each of 2 hours and 5 hours after theinitiation of the polymerization, 0.7 part of sodium oleate was added,each of 2 hours and 7 hours after the initiation of the polymerization,0.2 part of paramethane hydroperoxide was added, followed bypolymerization for 20 hours. As a result, hollow butadiene rubber latex(R-2) having a porosity of 30% and particle size of 0.12 mu·m wasobtained.

Synthesis, coagulation, heat treatment, dehydration, drying into powder,blending, molding and evaluation were conducted in the same manner as inExample 1, except for using (R-2) instead of (R-1). Results are shown inTable 1.

COMPARATIVE EXAMPLE 1

In a pressure polymerizer, 0.4 part by weight of tripotassium phosphate,0.2 part by weight of Na salt of a beta-naphthalinesulfonicacid-formalin condensate, 0.002 part by weight of ferrous sulfate (FeSO₄7H₂O), 0.004 part by weight of ethylenediaminetetraacetic acid 2Na saltand 1.9 part by weight of sodium oleate were mixed. To the resultingmixture was added 100 parts by weight of butadiene. After adjusting theliquid temperature to 40 degree C., 0.1 part by weight of paramethanehydroperoxide and 0.1 part by weight of sodium formaldehyde sulfoxylatewere added and polymerization was started at 40 degree C. Each of 5hours and 10 hours after the initiation of the polymerization, 0.1 partby weight of paramethane hydroperoxide was added and polymerization wasconducted for 20 hours. As a result, butadiene rubber latex (R-11)having a particle size of 0.08 mu·m was obtained.

After heating 77.5 parts by weight (as a solid content) of rubber latex(R-11) to 60 degree C., 0.0016 part by weight of ferrous sulfate (FeSO₄7H₂O), 0.004 part by weight of ethylenediaminetetraacetic acid 2Na saltand 0.2 part by weight of sodium formaldehyde sulfoxylate were added,and then a mixture of 16.5 parts by weight of methyl methacrylate, 3parts by weight of butyl acrylate, 3 parts by weight of styrene and 0.01part by weight of cumene hydroperoxide over 3 hours was continuouslyadded. Post polymerization was conducted for 1 hour to obtain graftcopolymer latex (G-11) having an average particle size of 0.9 mu·m.

The graft copolymer latex (G-11) was subjected to coagulation, heattreatment, dehydration, drying into powder, blending, molding andevaluation in the same manner as in Example 1 and the results are shownin Table 1.

COMPARATIVE EXAMPLE 2

Synthesis, post treatment, molding and evaluation were conducted in thesame manner as in Comparative Example 1, except that the amount ofsodium oleate used for polymerization of a butadiene rubber was changedto 1.2 parts by weight. The results are shown in Table 1.

COMPARATIVE EXAMPLE 3

Synthesis, coagulation, heat treatment, dehydration, drying into powder,blending, molding and evaluation were conducted in the same manner as inExample 1, except that 0.5 part by weight of allyl methacrylate wasadded as a crosslinking agent upon polymerization of a butadiene rubber.The results are shown in Table 1.

COMPARATIVE EXAMPLE 4

Synthesis, coagulation, heat treatment, dehydration, drying into powder,blending, molding and evaluation were conducted in the same manner as inExample 1, except that 1.0 part by weight of allyl methacrylate wasadded as a crosslinking agent upon polymerization of a butadiene rubber.The results are shown in Table 1.

COMPARATIVE EXAMPLE 5

Synthesis, coagulation, heat treatment, dehydration, drying into powder,blending, molding and evaluation were conducted in the same manner as inExample 1, except that 3.0 parts by weight of allyl methacrylate wasadded as a crosslinking agent upon polymerization of a butadiene rubber.The results are shown in Table 1.

TABLE 1 Comp. Comp. Comp. Comp. Comp. EX. 1 EX. 2 EX. 1 EX. 2 EX. 3 EX.4 EX. 5 Graft Hollow Hollow Conventional Conventional Hollow HollowHollow copolymer rubber rubber rubber rubber rubber rubber rubber RubberSeed diameter 0.015 0.025 0.015 0.015 0.015 (mu.m) Seed amount 2 2 2 2 2(parts by wt.) Rubber cross 0 0 0 0 0.5 1 3 linking agent ALMA(%) Rubber50 30 0 0 45 45 40 porosity(%) Rubber 0.08 0.12 0.08 0.12 0.08 0.08 0.08particle size(mu.m) Graft 0.09 0.14 0.09 0.14 0.09 0.09 0.09 particlesize(mu.m) Izod impact 13 30 8 18 8 6 5 resistace of molded article(23degree C.) Kg/cm/cm² ALMA: allyl methacrylate

The Izod impact resistance of a molded article tends to be higher whenthe particle size of a graft copolymer becomes larger. When the particlesize is same, a graft copolymer containing a butadiene rubber which ishollow in the form of a latex exhibits high Izod impact resistance. Evena graft copolymer containing a butadiene rubber which is hollow in theform of a latex has lowered Izod impact resistance when a crosslinkingagent is contained in the rubber so that it is preferred not to use thecrosslinking agent.

INDUSTRIAL APPLICABILITY

The present invention relates to a resin composition comprising arubber-containing graft copolymer and a thermoplastic resin and havingexcellent impact resistance.

1. A thermoplastic resin composition comprising: a graft copolymer and athermoplastic resin, the graft copolymer being obtained by polymerizingmonomers in a presence of hollow rubber particles; the monomersaccounting for 10 to 70 parts by weight of the graft polymer, andcontaining a vinyl monomer and another monomer; the vinyl monomeraccounting for 60 to 100% by weight of the monomers and being selectedfrom the group consisting of a (meth)acrylate ester compound, anaromatic vinyl compound and a vinyl cyanide compound, and the othermonomer accounting for 0 to 40% by weight of the monomers and beingcopolymerizable therewith; the hollow rubber particles accounting for 30to 90 parts by weight of the graft copolymer having a porosity of 3 to90% in a form of a latex, and being obtained by polymerizing apolymerization component without using a crosslinking agent, thepolymerization component comprising a butadiene.
 2. The thermoplasticresin composition according to claim 1, wherein the thermoplastic resinis a vinyl chloride resin comprising 50% by weight or more of vinylchloride.
 3. The thermoplastic resin composition according to claim 1,wherein the hollow rubber particle is obtained from the hydrophilic seedpolymer having a particle size less than 0.04 μm, the hydrophilic seedpolymer accounting for 0.5 to 20% by weight based on 100% by weight ofthe hollow rubber particle including the seed hydrophilic polymer.